Optical sensing of mechanical vibrations is a subject of interest in a variety of fields for many different applications. Optical vibration sensors are used to detect ultrasonic signals in different non-contact inspection of vibrating surfaces, non-destructive testing of engineering products, for non-invasive monitoring of medical parameters, for a variety of scientific investigation of delicate or inaccessible materials and devices, and for other purposes. Optical detection methods offer many advantages, in particular, non-contact, wide-bandwidth and high sensitivity. Optical detection methods, however, suffer from high sensitivity to environmental effects, such as instability and vibrations in the test arrangement and temperature variations. A further difficulty relates to detection of vibrations of optically scattering surfaces or media. The scattering introduces a strong modulation of the wavefronts of the optical probing beams which, in many cases, randomize the signal components that average out to generate very poor signal-to-noise ratios. Different methods have been devised to overcome the effects of such scattering, including Fabry-Perot interferometry, real-time holographic interferometry, phase conjugation of the scattering waves and various methods for time-domain or spatial domain filtering of the scattering effects. All of these methods are complex, often require careful setup or calibration procedures, involve quite cumbersome sensing components, and are by and large relatively expensive.
FIG. 1A schematically illustrates the per-se known knife-edge technique (KET) for detecting surface perturbations. A collimated probing light beam 2 is typically focused by a lens 4 onto a small spot on the surface 6, of interest. The reflected beam 2 is partially blocked with a knife-edge 10 and is incident on a photo-detector 12. Perturbation of the surface 6 to a new location 14 shifts the redirected or reflected beam 16 to a new location 18, as indicated by the broken line in FIG. 1A. Such shift necessarily modifies the portion of the beam 16 blocked by the knife-edge 10. Schematically illustrated in FIG. 1B is the portion of the original, unperturbed, beam 16 passing the knife-edge 10 and the location 18 of the reflected beam 16 when the surface is perturbed. Consequently, there is a change in the total intensity incident on the photodetector 12. In other words, perturbation of the surface 6, introduces a corresponding variation in the light intensity reaching the photodetector 12, which generates a corresponding electronic signal. Once in the electronic phase, the signal is amplified and presented on a suitable display or input to a processor (not shown).
While FIG. 1A presents detection of perturbation in the position of a surface reflecting the probing beam, the KET is equally suitable for detecting perturbations in the propagation direction of the probing light beam through bulk transparent media, i.e., variations in a refractive index due to variation in air pressure in a wind tunnel, and refractive index modulation due to acoustic waves.
The KET is used for a variety of probing beam perturbations, such as detection of vibrations of surfaces, detecting acoustic and ultrasonic waves and detection of dynamic variations in a refractive index in transparent media. This technique is, however, essentially limited to specular or nearly specular surfaces in the reflection mode, or non-scattering media in the transmission mode. When applied to a rough surface, the probing beam is scattered generating an irregular pattern in the plane of the knife-edge and on the photodetector. In general, such patterns have irregular intensity and phase variations. Consequently, when the surface moves and perturbs the reflected beam, the intensity and phase of the generated pattern are disturbed in an irregular fashion. For example, the intensity in some areas increases, while in other regions, it may decrease. Similarly, the phase may increase in some areas but decrease in others. As the photodetector integrates these changes, which become randomly self-canceling, a very low overall signal, which may even be indiscernible, results.
In the interest of brevity, the description in the following is limited to surface reflection configurations straightforwardly extended to the case of detection of perturbation-in-bulk, in transmission.
FIG. 2A schematically illustrates a known approach to overcome the problem of random variation of the light pattern reflected off a rough surface. A probing beam 2 is focused to a small spot on the surface 6 by a lens 4. To counter the effect of the random nature of the reflected pattern, an aperture 20, e.g., an aperture in a knife-edge disk, is introduced. The diameter of the aperture 20 is chosen to be in the order of the reflected pattern variation period, so that a small number of pattern “spots” 22 (FIG. 2B) is allowed to pass through the aperture 20 and reach the photodetector 12. In this case, the small number of spots 22 reduces the irregularity of the intensity and phase variations within the reflected pattern. Therefore, when the pattern is perturbed by a shift of the original surface 6 to a different position 14, the overall summed signal is still appreciable, even if some of the contributions are of opposite signs.
The embodiment illustrated in FIGS. 2A and 2B may present some difficulties, as follows:    1. The choice of a suitable aperture 20 will depend on the characteristics of the surface roughness, i.e., the longer the characteristic spatial wavelength of the roughness, the larger an aperture 20 would be required for optimal performance. Indeed if the aperture 20 is too small, so as to pass only a portion of a “spot”, the detected signal will be very weak;    2. Even though a small number of components 26 is detected, the components are still partially self-canceling, and thus, the overall signal reduces with respect to detection on a specular surface, and    3. Typically, a small aperture 20 is required in highly scattering surfaces; the resulting overall light intensity that reaches the detector 12 will therefore also be small, thereby introducing a practical difficulty of detecting relatively low light levels.
GB 2 052 048 corresponding to U.S. Pat. No. 4,275,963 discloses a method and system for sensing acoustic energy responsive deformation of a workpiece surface in a contact-free manner by optical means using a multi-wavelength laser which illuminates the surface at which deformation is expected. The reflection light energy is transmitted to an optical interferometer and thereafter separated into the distinct wavelengths. Each light signal corresponding to a specific wavelength is converted to an electrical signal, which is rectified, and averaged.
WO 03/089955 discloses an apparatus and method for identifying a remote target illuminated with radiation generated by a laser. The light scattered by the target is modulated in phase by the surface vibrations of the target. A portion of the scattered radiation is collected by multiple optical receivers and demodulated by a phase demodulator to generate a signal proportional to the vibrational displacement of the remote target. The radiation scattered by the remote target will also include laser ‘speckle’, generated when radiation is scattered by a rough solid surface. This speckle can generate errors in the signal demodulated, which can in turn cause identification errors.
As further explained on page 5, lines 12ff signal light from the target surface is mixed with light from a frequency shifter, resulting in optical interference, which gives rise to intensity modulation that is detected by different photodiodes.
WO 86/06845 discloses a laser velocimeter for measuring the relative speed of a surface and a source of coherent light directed thereto comprising optical and electrical means for generating two electrical signals one of which corresponds to the content of the speckle pattern produced by illumination of the surface for obtaining a difference signal from the two electrical signals. The centre frequency of the signal spectrum of the difference signal is determined to generate an electrical signal indicative of the relative velocity of the surface and source.
US20050210982 discloses laser vibrometers for detecting surface vibrations of a remote mass excited with one or more beams. Each vibrometer generates a signal indicative of the surface vibrations which is stored, reversed in time, and applied to phase modulate an exciter beam that is then impinged onto the mass.
The present invention proposes the use of simple, low-cost, compact components and offers both high sensitivity and good immunity from environmental effects.